Isothermal annealing effects on β-relaxations and crystallization behaviors in amorphous GeTe

A secondary (β) relaxation process is the dominant source of atomic dynamics below T g in many glass forming systems. Recent studies reported the presence of β-relaxations in amorphous phase-change materials (PCMs). They showed that suppressing β-relaxation via annealing in Ge 15 Sb 85 can effectively slow down its crystallization kinetics. Yet, when Sb is replaced by Te, a similar annealing protocol has little effect on the Te-rich alloy Ge 15 Te 85 , which is a good glass former. Here, we investigate amorphous GeTe that is a Sb-free PCM, but with faster crystallization kinetics than Ge 15 Te 85 . GeTe is a typical PCM, i.e., crystallizes very rapidly. Using powder mechanical dynamic spectroscopy, we observe a clear reduction of the excess-wing in the loss modulus upon isothermal annealing of amorphous GeTe, indicating a suppression of its β-relaxation. Combining ultrafast calorimetric analysis and time-resolved optical reflectivity measurements shows that, whereas as-deposited GeTe exhibits stochastic crystallization behavior, annealed samples crystallize more slowly but also with reduced stochasticity. Synchrotron X-ray scattering experiments reveal increased Peierls-like distortions in the amorphous structure after annealing, and demonstrate that, even if annealing introduces nucleation sites, it nonetheless slows down crystallization kinetics. These finding suggests that, in the annealed GeTe, crystallization is limited by crystal growth rate, which is retarded through a process likely associated with the suppression of β-relaxation.